This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. This subproject, ongoing for 15 years, addresses the use of MS methods to detect and characterize photodamage to DNA. C to T and CC to TT transition mutations are the most frequent mutations observed in skin cancer. A deamination hypothesis was proposed to address the generation of those mutations. In this hypothesis, a cytosine or 5-methylcytosine photoproduct deaminates to form a uracil or thymine photoproduct. Those uracil or thymine moieties in the photoproduct form Watson-Crick base-pair preferentially with adenine. After two steps of DNA replication, C to T transition mutation results.